The invention relates to bending wave loudspeakers which are often flat panel loudspeakers.
Flat panel speakers and indeed most conventional speakers until recently have operated intentionally in a pistonic regime, but natural break-ups invariably caused unwanted interference with the intended mode of operation. Notably cone type loudspeakers suffer from a variety of shortcomings including limited bandwidth and beaming at the higher range of their operative bandwidth a phenomenon which is diaphragm size dependant.
Other flat panel speakers are known which use a stretched membrane type of diaphragm and which operate through propagation of constant speed waves across the panel surface. In this case too, natural dimensions, areal mass density and the membrane tension primarily decide the nature and extent of modality in the panel, although for most materials inherent membrane damping tends to reduce modality to some extent. This type of loudspeaker has some desirable acoustic properties, notably wide radiation pattern and reasonably wide bandwidth. However by the nature of the construction such loudspeakers are very difficult to make in consistent quality.
More recently, bending wave loudspeakers have been developed, see for example EP 0541,646 of Heron and EP0847661 of Azima et al, which rely on either a multi-modal or a distributed-mode operation. In both these cases, especially in the latter case known as DML, which substantially defines the basis of a new form of wide-band loudspeaker using natural plate resonance to reproduce acoustic output, the modality is caused by the finite panel size and the ensuing build-up of modes primarily due to the dimensions of the panel, bending stiffness, and areal mass density of the material. It has been shown that this type of loudspeaker can exhibit desirable acoustic properties that were not possible to achieve in the prior art. In the case of distributed-mode loudspeakers, the lower frequency range can in some circumstances suffer from sparse modality which limits, at least for high-fidelity purposes, the speaker in its lower frequency range of operation.
It is the intention of the present invention to achieve a more effective use of bending waves for reproduction of sound especially in the lower operating range of the loudspeaker. It is an objective of this invention to avoid altogether or at least reduce the modal behaviour of the panel, either throughout the operating range or at least in the lower frequency range of operation. Ideally, the panel should behave as if it were infinite in sizexe2x80x94that is no energy is reflected from the boundaries, despite its finite physical size. The core idea of the present invention is that the imposition of an acoustic aperture onto a conceptually infinite panel results in a net acoustic power available in the far field of the panel at below the coincidence frequency, and also above it.
It is well documented that an infinitely large panel operating in bending plane wave radiates little or no acoustic energy below its coincidence frequency (frequency at which speed of sound in the panel reaches that of its surrounding air (fluid)) To overcome this limitation, a distributed mode loudspeaker in effect imposes a finite mechanical aperture onto an infinite panel (by its finite size and boundary conditions), thus creating a modal object to achieve this effect. The effect of this aperture is to either present a zero (clamped edge) or infinite (free edge) mechanical impedance to the panel and therefore instigate reflections in order to build up natural resonant behaviour in the panel.
In contrast, the present invention stipulates substantially terminating the panel structure at the panel boundaries, ideally to absorb incident bending wave energy. This is tantamount to an infinite panel with a finite acoustic aperture imposed on it. This is a significant departure from the prior art and in fact an antithesis to a modal object.
Thus, according to the invention, there is provided a loudspeaker comprising a panel which is sufficiently stiff to support bending waves, the panel having a boundary, a transducer mounted to the panel to apply bending wave energy in the form of dispersive travelling waves thereto at a first location in response to an electrical signal applied to the transducer to cause the panel to vibrate and radiate an acoustic output, the loudspeaker having a frequency range extending from a lower frequency to a higher frequency and the panel having a stiffness giving a coincidence frequency above the lower frequency, and comprising means on or associated with the panel at a second location to attenuate travelling bending waves in the panel to prevent or at least substantially to moderate panel resonance, the attenuating means acting in the manner of an acoustic aperture over an infinite bending plate.
The attenuating means may comprise mechanical impedance means at a panel boundary and matched to the mechanical impedance of the panel to provide absorption of bending wave energy reaching the panel boundary. The attenuating means may be located on or in the panel to attenuate bending wave energy before it reaches the panel boundary. The attenuating means may be frequency dependent. The frequency dependence may be such that higher frequencies of bending wave energy are reflected from the panel boundary.
The mechanical impedance means may extend round substantially the entire panel boundary.
The attenuating means may comprise a predetermined stiffness or structural mechanical impedance profile across the panel.
The mechanical impedance means may increase bending wave energy absorption at, or bending wave energy transfer across, at least a portion of a boundary of the panel.
The attenuating means may provide a non-uniform or varying mechanical impedance profile across at least a portion of the panel.
The attenuating means may provide an increase in attenuation towards a boundary of the panel.
The attenuating means may provide a reduction in attenuation towards the centre of the panel.
The attenuating means may have a mechanical impedance which is substantially matched to a mechanical impedance at an interface between at least a portion of the panel and a frame for the panel.
The attenuating means may comprise a variation in panel thickness or density across at least a portion of the panel.
The attenuating means may comprise a layer over one or both surfaces of the panel and/or incorporated within the panel.
The bending wave panel may comprise a termination provided at or towards at least a portion of a panel boundary.
The termination may have a predetermined mechanical impedance for substantially terminating a mechanical impedance of at least a portion of the panel to an impedance of a portion of a frame for the panel. The termination may have a predetermined mechanical resistance for reducing the energy of a bending wave moving towards a panel boundary.
The first location may be at the panel centre.
From another aspect the invention is a microphone comprising a panel which is sufficiently stiff to support bending waves, the panel having a boundary, a transducer mounted to the panel to produce an electrical signal in response to bending wave energy in the form of dispersive travelling waves in the panel caused by incident acoustic radiation, the microphone having a frequency range extending from a lower frequency to a higher frequency and the panel having a stiffness giving a coincidence frequency above the lower frequency, and comprising means on or associated with the panel to attenuate travelling bending waves in the panel to prevent or at least substantially to moderate panel resonance, the attenuating means acting in the manner of an acoustic aperture over an infinite bending plate.
From a further aspect, the invention is an acoustic device comprising a panel which is sufficiently stiff to support bending waves, the panel having a boundary, the device having a frequency range extending from a lower frequency to a higher frequency and the panel having a stiffness giving a coincidence frequency above the lower frequency and comprising means on or associated with the panel to attenuate travelling bending waves in the panel to prevent or at least substantially to moderate panel resonance, the attenuating means acting in the manner of an acoustic aperture over an infinite bending plate.
There are two principal methods of achieving the objective of the invention. A bending wave object of the present invention, with the desired action, may use a combination of the two techniques.
Ideally the panel system should have a structure with a mechanical impedance all around its boundaries designed to terminate the mechanical impedance of the panel. This will result in the full absorption of the bending wave energy reaching the boundaries.
An alternative approach would be for the panel to incorporate sufficient and appropriate damping, either intrinsic or added on by the application of damping material to its surface or internal structure, to absorb the bending wave energy gradually as it radiates out from the exciter(s). Thus by the time the waves reach the boundaries they would have lost most or all their energy and hence cause little or no reflections.
In practice, a combination of the above two techniques may be used to achieve the desired performance. In both cases, the damping structure may be deliberately designed by specifying the material and/or the structure of it to be frequency dependent in order to achieve a given acoustic targetxe2x80x94for example it may be desirable for the panel to become modal at higher frequencies.
According to both of the above mentioned approaches, the damping can be incorporated in or around the panel so as to significantly reduce the energy of the bending waves at, or as the waves approach, the periphery of the panel. However, neither of the above mentioned approaches involves the incorporation of damping such that the efficiency of the panel is unduly compromised. Damping of a desired kind can be achieved by having a predetermined stiffness or structural impedance profile across the panel or by the inclusion of forms of edge termination.
In one form of the present invention, a bending wave panel is provided with a medium for reducing the reflection of bending wave energy from at least a portion of a boundary of the panel.
In another form of the bending wave panel of the present invention, there is a gradual reduction or increase in damping or impedance across a panel.
In another form of the bending wave panel of the present invention, a reduction or increase in damping or impedance across a panel is substantially linear.
In another form of the bending wave panel of the present invention, a reduction or increase in damping across a panel is substantially non-linear and can be, for example, exponential.
In another form of the present invention, a bending wave panel comprises a medium which presents an impedance to a bending wave in the panel.
References herein to impedance include references to reactance and/or resistance.
References herein, both explicit and implicit, to acoustics or sound include references to infrasound and ultrasound.
The present invention is not limited to application in loudspeakers but can also be applied to other acoustic transducers such as microphones, couplers and the like.